1. Field of the Invention
The present invention relates to a transponder and a method for wireless data transmission.
2. Description of the Background Art
Contactless identification systems or radio-frequency-identification (RFID) systems typically include a base station, a reading device, or a reading unit and a plurality of transponders or remote sensors. The transponders or their transmitting and receiving devices typically do not have an active transmitter for data transmission to the base station. Such inactive systems are called passive systems when they do not have their own power supply, and semipassive systems when they have their own power supply. Passive transponders draw the power necessary for their supply from the electromagnetic field emitted by the base station.
For data transmission between the transponder and the base station, for example, for a programming operation of the transponder, the transponder has an interface of a specific interface type, which is compatible with the corresponding interface type of the base station. The interface types can be divided, in a preliminary rough grouping, into contact and contactless types.
The interface types with which the data transmission occurs contactless or contact-free differ, inter alia, in the operating or carrier frequency used for the data transmission, i.e., the frequency transmitted by the base station. Frequently used frequencies are, for example, 125 kHz (LF range), 13.56 MHz (RF range), a frequency range between 860 MHz to 960 MHz (UHF range), and a frequency range greater than 3 GHz (microwave range).
Another differentiating feature of the different interface types is the type of coupling between the specific interfaces of the transponder and the base station. In this case, inter alia, an inductive or magnetic coupling and a far-field coupling are differentiated. Described in simplified terms, in inductive or near-field coupling, an antenna coil of the base station and an antenna coil connected to the input circuit of the transponder form a transformer, which is why this type of coupling is also called a transformer coupling. In inductive coupling, a maximum distance between the transponder and the base station is limited to the near field of the employed antenna. The near-field range is substantially established by the operating frequency of the interface.
A modulation is usually used in inductive coupling for data transmission from a transponder to a base station; in this regard, see, for example, Finkenzeller, Chapter 3.2.1.2.1 “Load Modulation.”
For data transmission from the base station to the transponder, the base station in inductive coupling usually transmits a carrier signal with a frequency in a frequency range of 50 kHz to 250 kHz. To begin the data transmission, the base station via amplitude modulation of the carrier signal first generates a short field gap or a so-called “gap”; i.e., the amplitude of the carrier signal is dampened or attenuated briefly, for example, for about 50 μs to 400 μs, or totally suppressed.
Characters which are transmitted subsequent to the initiation of the data transmission by the base station are encoded by associated durations between temporally successive field gaps. A first character value is hereby assigned a first duration and at least one second character value is assigned a second duration. To decode the transmitted characters, the transponder determines the specific durations between the field gaps and determines the value of the transmitted character from the determined duration.
For error-free data transmission or decoding of the characters, it is necessary that the signal courses generated by the base station and received by the transponder by inductive coupling have established maximum tolerances, for example, in regard to their time course and/or employed level.
To increase the achievable ranges between base station and passive transponders, the quality of a parallel resonant circuit, which is formed by the antenna coil and a capacitor connected parallel thereto, is increased in order to enable the supplying of the passive transponder from the field transmitted by the base station at greater distances as well. The reduced damping of the resonant circuit has the effect that at a field gap a coil voltage or a voltage of the parallel resonant circuit of the transponder declines more slowly than in the case of a resonant circuit with a lower quality and therefore higher damping. Because the field gap in the transponder can be detected, however, only when the coil voltage or a voltage obtained from the coil voltage by rectification has declined below a settable potential, field gaps can be detected in a delayed manner in comparison with a resonant circuit with a lower quality. This has the result that the durations of the field gaps detected in the transponder are shortened and the durations between the field gaps are lengthened. This change in the timing of the signal courses detected in the transponder is influenced directly by the quality of the resonant circuit. Because the timing must lie within predefined limit values, however, the signal transmitted by the base station should have a timing, which depends on the quality of the resonant circuit of the transponder, because otherwise transmission errors can arise.